Defrost control



Nov. 27, 1962 w. A. ARZBERGER DEFROST CONTROL Filed March 1, 1957INVENTOR. WILLIAM A. ARZBERGER.

ATTORNEY.

FIG. 3

3,065,608 DEFROST CONTROL William A. Arzberger, North Syracuse, N.Y.,assignor to Carrier Corporation, Syracuse, N.Y., a corporation ofDelaware Filed Mar. 1, 1957, Ser. No. 643,397 4 Claims. (Cl. 62$ti) Thisinvention relates to a defrosting control for an evaporator of arefrigeration system.

In the utilization of refrigeration systems of thecompressor-condenser-evaporator type, one of the major problems involvedin obtaining a high order of efficiency is that of removing the coatingof frost formed on the evaporator as moisture deposited thereon freezes.For illustration the evaporator in a domestic refrigerator in coolingthe air circulating within the refrigerator, has deposited on thesurface thereof, a coating of frost during its operation. This frostoccurs as moisture is precipitated from the air circulating within thecompartment as the evaporator cools the air to a temperature below itsdew-point temperature. The temperature of the evaporator being below thefreezing point of Water, the moisture deposited on the evaporatorsurface freezes to form a frost formation. This frost is objectionablein this location because it acts as an insulator between therefrigerated evaporator and the air inside the cabinet. Variousautomatic devices are currently used for removing this frost. Thearrangement proposed here consists of inactivating the refrigerationsystem evaporator when the evaporator temperature is reducedsufficiently to cool the air to a desired low temperature andreactivating the evaporator when all of the ice on the evaporator hasbeen melted by means of the Warming air passing over it. This type ofcontrol of the evaporator is ordinarily accomplished by a thermostathaving a control bulb attached to the evaporator surface.

Placement of the bulb on the evaporator has been found to havedisadvantages. First, the cut-in temperature of the thermostat must berather high to insure complete defrosting. Second, as this bulb issensing metal temperatures and is not responsive to the air temperature,the cabinet air temperature varies considerably as the amount of heatleakage through the cabinet insulatioii varies in response to ambientchanges. Third, in the use of combination cabinets with a frozen foodsection, and another section for normal above freezing storage,placement of the control bulb on the freezing section evaporator,results in excessively low refrigerator temperature, freezing ice orfood in the refrigerator compartment.

This invention has for its chief object the provision of anautomatically operable defrosting control for effectively removing thecoating of frost from the evaporator during those periods when it isnecessary to free the surface of the coil of the frost in the interestof obtaining more effective operation of the evaporator.

An additional object of the invention is the provision of a method ofdefrosting the evaporator of a refrigeration system which ensures acomplete removal of the frost before the defrosting action isterminated.

In attaining the objects of this invention, it is proposed that a bulb,forming a part of a conventional thermalresponsive unit which controlsthe operation of a switch in a defrost circuit, be supported in spacedrelation to the evaporator so as to be influenced by the run off ofmoisture that occurs during the defrosting operation. Thethermal-responsive unit is used to sense a temperature of a value toindicate the necessity for defrost, and to institute the defrostingaction. During the defrosting action, the frost on the surface of theevaporator coil is melted and flows from the evaporator through a path3,065,608 Patented Nov. 27, 1962 that includes the bulb. Thus the bulbsenses a temperature in the vicinity of 32 which is sufficiently low sothat termination of the defrosting action and resumption of the normaloperation of the evaporator is precluded. However, once the moisture hasdrained from the evaporator coil, the bulb senses an air temperaturesufficiently high to indicate the need for termination of the defrostingcycle and resumption of the normal freezing cycle.

These objects and features of the invention as well as other objects andfeatures will be apparent from a consideration of the ensuingspecification and drawings in which FIGURE 1 is a diagrammatic view of arefrigeration system with which the defrosting control forming thisinvention may be used;

FIGURE 2 is an isometric view of an evaporator equipped with a portionof the defrost control;

FIGURE 3 is a detail illustrating one mannerin which the defrost controlbulb of the thermal-responsive system may be connected to theevaporator; and

FIGURE 4 is a view similar to FIGURE 3 showing another arrangement forsubjecting the defrost control bulb to the moisture flowing from theevaporator during the defrosting cycle.

Referring more particularly to the drawings, there is shown oneembodiment of the invention merely for the purpose of describing theinvention. FIGURE 1 is a diagrammatic View of a refrigeration systemwhich may be employed in a refrigerator of the domestic type whereinthere is provided a first compartment, for cooling foods, served by afirst evaporator and a second compartment, for maintaining foods in afrozen condition, which is served by a second evaporator. Inrefrigeration systems of this type, the two refrigerators operate atdifferent temperature levels. This invention is concerned with thedefrost of the evaporator serving the compartment used for cooling thefoods. This is normally referred to as the refrigerator evaporator.

In the system shown, refrigerant is discharged in the gaseous phase frommotor compressor 10 to the condenser 12 through a discharge line 11.Gaseous refrigerant is liquefied in the condenser 12 and flows throughline 13 to a freezer capillary 14 and into freezer evaporator 15.Refrigerant flowing from the 'line 13 also flows through a refrigeratorcapillary 16 in series with a refrigerator evaporator 17. From eachevaporator, refrigerant in the gaseous and/ or liquid phase flows to theaccumulator 18 where the gaseous portion of the refrigerant flow isdelivered to the compressor through suction line 19.

Refrigerant flows continuously through the circuit described so thatheat sufficient to cause vaporization of the refrigerant in the twoevaporator sections is absorbed from the air flowing within thecompartment being served by each evaporator. The air circulated withinthe refrigerator compartment or the compartment served by the evaporator17 is cooled below its dew-point so that moisture from the air isdeposited on the surface of the evaporator unit 17. The moisture sodeposited is con verted into frost because of the surface temperature ofthe evaporator which is below 32.

This accumulation of frost builds up over a certain length of time to apoint where the operating efficiency of the evaporator is seriouslyimpaired. In order to restore the evaporator to its original efiicientlevel of operation it is necessary to remove the frost in an actionnormally referred to as a defrost action. This invention contemplatesthe removal of this frost automatically and likewise, contemplates theresumption of the operation of the refrigerant evaporator once thecoating of frost has been removed.

To this end, there is located in close proximity to the evaporator, atrough 19; serving to collect the moisture flowing by gravity from theevaporator 17 during the defrosting action. The trough is shown as beingsubstantially U-shaped and having a pitch or inclination such thatmoisture flows down the opposed legs of the trough to the connectingportion and from the connecting portion through a drain 20" where it isdisposed of in any one of a number of conventional manners. 'ReferringtoFIGURE 2, there is shown a type of evaporator unit that is used in thecooling compartments of a domestic refrigerator of the kindcontemplated. The evaporator coil 17 is secured to the back portion of aplate 38 which in turn may be secured to the walls of a compartment of adomestic refrigerator. In the interests of clarity, the refrigeratorconstruction has not been illustrated. However, it will be evident thata conventional refrigerator having one or more compartments for thereception of food may be employed. The compartment normally containsopposed side walls as well as top, bottom and rear walls. The plate 38is secured in fixed relation to the side walls and back of thecompartment. Likewise, the trough 19' is also connected as by fasteningelements 20 and insulating bumper means 21 to the walls of thecompartment.

It is contemplated that the defrosting of the evaporator 17 occur by'virtue. of the operation of a solenoid valve located in the lineconnecting the evaporator with the accumulator 18. When a condition issensed necessitating a need for defrost action, solenoid valve 40 isclosed so as to prevent the flow of refrigerant through coil 17.Thisinactivates the coil and permits the temperature of the air in thecompartment being served by the coil to rise. Oncethe temperature in thecompartment rises above 32, the fro-st accumulation on the coil willmelt and drain off of the evaporator into the, trough 19 and be carriedaway through the drain 20'.

Referring to FIGURE 1, it will be noted that the coil of solenoid valve40 is disposed in a circuit including a switch 26 operable under theinfluence of a thermalresponsive system 22. The thermal-responsivesystem includes a bulb 23, a bellows 24 and a capillary 25 conmeetingthe bulb and the bellows. The system, as described, is a conventionalunit wherein a thermal-responsivfe'fill is enclosed within the threeelements 23, 24 and 25.

In order to effectively attain the objects of this invention, it isproposed that the bulb 23 of the thermal system 22 be placed in relationto the evaporator coil so that it'is subject to the run off or drain ofmoisture from the 'coil. This may be accomplished in a mannerillustrated in FIGURE 3 wherein a strap 28, fixed to the evaporatorplate, 38, supports the bulb in such a manner that it is subjectedto thetemperature of the air circulating about the evaporator as well as themoisture which drains 013? of the evaporator. Another arrangement foraccomplishing this is shown in FIGURE 4. Here, strut numbers 30 supportthe bulb in space relation to the evaporator coil. The struts 30 aresupported from the upper ends of the wall portions of the trough 19.

In refrigeration systems of the type described here, there is normallyincluded an arrangement for defrosting or limiting the temperature ofthe freezer evaporator. Accordingly, a second thermal-responsive unit32, including a bulb 33, secured to the freezer evaporator, a bellows34, and a connecting capillary 35, is shown. When the temperature of thefreezer coil drops to a predetermined low'leve l, indicating theexistence of a coating of frost on the freezer evaporator coil, bellows34 contracts to open switch 36 and interrupt a circuit through thecompressor motor 10.

Considering the operation of this invention the refrigeration systemoperates in its normal intended manner until a predetermined coating offrost appears on the evaporator. This coating of frost is determined bythe temperature sensed by the bulb 23, in a following man.-

ner. As shown in FIGURE 3, the bulb 23, is subject not only to the airtemperature in the immediate vicinity of the evaporator but is alsosubject to the temperature of the refrigerator by conduction through theconnecting support clip 28. This latter effect, however, is of asecondary significance because the strap is composed of a materialhaving a high resistance of heat transfer. When the bulb senses atemperature of about 13 F., the thermal responsive fill in the thermalsystem cools sufficiently to reduce the pressure in the unit and causethe bellows to contract and close the switch 26 completing a circuitthrough the coil of the solenoid valve 40. This action closes thesolenoid valve which in turn stops the flow of the refrigerant throughthe coil 17. This action has the effect of de-activating coil 17 causingthe temperature within the compartment being served by the evaporator torise. Once the evaporator temperature rises over 32, the frost on thecoil melts and drains by gravity into the trough 19 flowing over bulb 23in the process. It will be apparent that the air temperature of theevaporator compartment will be above 32. Inasmuch as the cut-in point ofthe bulb 23 may be set for a temperature of about 37 F., it is obviousthat the bulb must be subjected to some temperature other than airtemperature in order to insure complete defrosting of the coil. With theconstruction shown, it is obvious that the coil is being, subjected to aflow of moisture having a temperature below the cut-in point of 37. Oncethe coil has been completely defrosted, and the flow of water has beenterminated, the bulb 23 then senses the air temperature within thecompartment. By this time, the temperature of the air flowing over thebulb from the evaporator has risen above 37 causing thethermal-responsive fill in the unit 22 to expand and expand the bellows24 to open the circuit and de-energize the coil in solenoid valve 30.Once again refrigerant is free to flow through the evaporator 17 and thesystem once again resumes a normal operation.

Thus by placing the bulb in the position shown in FIG- URE 3, themoisture draining from the coil during the defrosting operationdetermines the temperature sensed by the bulb. Once the moisture flowterminates, the air within the compartment flows over the bulb and itstemperature determines the temperature sensed by the bulb. Flow of airwithin the compartment over the evaporator obtains by virtue ofconvection. When the bulbis placed as shown it is subjected to thecoldest air (direct flow from the evaporator) in the compartment. Thearrows on FIGURE 3 indicate the direction of a portion of air flow inthe compartment. Under this arrangement a wider temperature differentialbetween the cut-in and cut-out points may be selected. With such a rangea less expensive thermal unit may be used. Also the bulb is locatedwhere the air is moving relatively fast as compared to the movement ofthe air stream in other parts of the compartment. This allows a fasterresponse to be obtained, an important feature when a load is placed onthe freezer evaporator 15. Under these latter circumstances the backpressure in the suction line rises causing a corresponding rise insuction temperature. This rise in temperature likewise causes a rise inthe evaporator temperature so that when the bulb 23 is placed directlyin contact with the evaporator surface, the air temperature coulddecrease while the evaporator temperature was still above the defrostcut-out point.

Thus it will be obvious that an automatic defrost control arrangement isprovided for insuring complete removal of frostfrom the coil before thedefrosting action is terminated and the normal operation of theevaporator is restored.

Other arrangements and constructions capable of performing thisinvention will suggest themselves to those skilled in the art. Thedescribed operation is equally applicable to a single compartmentrefrigerator,

wherein. the compressorstarts and stops by this thermoe,ees,eos

stat. However, the above description is offered for the purpose ofillustrating the operation of an embodiment of the invention and is notintended to limit the invention as defined in the appended claims.

I claim:

1. In a refrigeration system for cooling air circulating within anenclosure and including a compressor, a condenser and an evaporator overwhich said air circulates, means for removing frost from the surface ofsaid evaporator; trough means disposed beneath said evaporator toreceive moisture draining therefrom during defrost; and control meanscomprising a thermal responsive element supported in spaced relation tosaid evaporator so as to be responsive to the temperature of the aircirculating within the enclosure as well as the flow of moisture fromthe evaporator to the trough as the evaporator defrosts whereby saidcontrol means is operative to effect defrost as long as moisture drainsfrom said evaporator and is operative to terminate defrost in theabsence of drainage and in the presence of the relatively warm aircirculating over the evaporator as drainage ceases.

2. The invention set forth in claim 1 wherein said thermal responsiveelement is suspended from said evaporator by a strap formed of amaterial having relatively high resistance to the transfer of heat.

3. The invention set forth in claim 1 wherein struts are providedsupporting said thermal responsive element in spaced relation to saidevaporator.

4. The method of defrosting an evaporator of a re frigeration systememployed to cool air circulating within an enclosure over saidevaporator which consists in the steps of interposing a thermal controlelement in spaced relation to said evaporator in the air stream fiowingover said evaporator, arranging said element so as to be responsive tothe flow of moisture from the evaporator as said evaporator defrosts,terminating flow of liquid refrigerant through the evaporator inresponse to a predetermined air temperature to raise the temperature ofthe evaporator to defrost same, continuing the defrost operation as longas the fiow of moisture over said thermal control element ensues, andterminating defrost of the evaporator in the absence of drainage ofmoisture from the coil and in the presence of an air temperature of apredetermined maximum value.

References Cited in the file of this patent UNITED STATES PATENTS1,516,739 Ketterer Nov. 25, 1924 2,135,875 Morse Nov. 8, 1938 2,479,011Kernper Aug. 16, 1949 2,687,020 Staeb-ler et al. Aug. 24, 1954 2,716,867Jacobs Sept. 6, 1955 2,729,074 Monroe Jan. 3, 1956 2,773,354 TillmanDec. 11, 1956 2,867,093 Simmons Jan. 6, 1959

